Ultimate Guide to Understanding Vaccinology in Dogs and Cats

About Vaccines

I’m going to try to make all this science stuff understandable, so please let me know how well I succeed. This is a crash overview course in vaccines and vaccinology that took almost a semester of vet school to learn. I promise not to go into that much detail unless you really want to (please say no LOL).  It’s not that important for this discussion anyway. Oh, and this is just about adaptive immunity. There’s a whole separate part of the immune system called the innate system that deals with diseases in a non-specific manner, but we don’t need to know about that right now.   

Let’s start with some definitions:

Antigen– anything that makes the immune system react. This could be a virus, a bacteria, or something that should be harmless like peanut butter or pollen.  

Pathogen– any antigen that is actually disease-causing (not peanut butter, that’s a hypersensitivity reaction, not a disease)

Antibody– a protein that attaches to an antigen to make the antigen ineffective, or that targets the antigen for destruction by other cells and proteins.

B-Lymphocyte– a type of white blood cell that produces antibodies

T-Lymphocyte– a type of white blood cell that destroys infected cells (cytotoxic T-cell)

Antigen Presenting Cells (APCs)– cells that engulf a pathogen, process it, and show the antigen to lymphocytes.

Passive immunity– maternal antibodies, passed along from the mother to the baby through the placenta and in milk.

Active immunity– the antibodies that the individual makes themselves by being exposed to an antigen

Memory cells– B lymphocytes that can quickly produce antibodies, or T lymphocytes which can quickly recognize and destroy infected cells in the body

Vaccination– a substance that is enough like an actual pathogen to trick the immune system into targeting the actual pathogen if it is ever seen.

Adjuvant- a substance added to a vaccine in order to stimulate a better immune response.

Sterilizing immunity– when a vaccine prevents an animal from showing any signs of disease and keeps the animal from being contagious to others

Non-sterilizing immunity– when a vaccine dramatically reduces the clinical signs and duration of illness for a disease, but the animal can be contagious to others

Naive Animal– any animal that has not yet had exposure to a particular antigen by infection or vaccination

Apoptosis- a process of programmed cell death, initiated by active cytotoxic T-cells

Exposure of a Naive Animal to an Antigen

The immune system quickly reacts to a new antigen to produce antibodies and neutralize the threat. Along with that initial response, the immune system prepares for subsequent exposures to the same antigens. After all, there is no point in winning the immunity battle if you can’t win the war!

I’m going to skip over a lot of how the APCs make helper T-cells and how those helper T-cells turn into other types of cells. Because honestly, it’s not that important. There are actually 5 different kinds of T-cells responsible for fighting different types of pathogens. Since we’re discussing vaccines, I’m only going to talk about the T-cells that kill intracellular pathogens such as viruses and bacteria.

Antigen-Presenting Cells

Antigen-Presenting Cells (APCs) include macrophages, dendritic cells, mast cells, some B-lymphocytes, and other types of cells. They live in tissues and circulate in the bloodstream. The APCs’ job is to find foreign material and make it obvious for other types of cells to “see” and react to. They do this by breaking down the foreign material and displaying the antigen all over the outside of their cell wall. At this point, we call them helper T-cells.

Helper T-Cells

Helper T-cells are the keys to both antibody-mediated and cell-mediated immunity. They can differentiate, or change, into one of 3 kinds of cells: cytotoxic T-cells, B-cells, or memory helper T-cells. Each of these different lines of cells has very specific jobs in the immune response.


On the antibody-mediated immunity side, B cells recognize the antigen, then turn into plasma cells and secrete specific antibodies. Antibodies are kind of flaggers that invite other cells and substances to destroy them along with the pathogen. Antibodies can also bind directly to the antigen and prevent the antigen from attaching to or infecting other cells. B cells also turn into memory B cells. Memory B-cells allow the immune system to skip all the previous steps and immediately turn into antibody-secreting plasma cells.  

Cytotoxic T-cells

Helper T-cells also produce cytotoxic T-cells. Cytotoxic T cells recognize the antigen and turn into active cytotoxic T cells. They find other infected cells in the body and destroy those cells by apoptosis. Cytotoxic T cells also make memory cytotoxic T cells, which turn into active cytotoxic T-cells quickly upon exposure to the same pathogen.

 Memory Helper T-Cells

Memory Helper T-cells are awesome because they can turn into memory B-cells or memory T-cells upon subsequent exposure to the same pathogen. All of the memory cells produce long-lasting immunity to certain antigens. The immune system protects the individual from disease even if there are no detectable antibodies present. Memory cells go into action almost immediately and can destroy an antigen quickly, often before the individual ever gets sick.

There is one catch to memory cells though: they are only effective against one particular antigen. If a new strain of a certain disease is significantly different, the memory cells may not recognize it as the same antigen. Exposure to H3N2 does not protect a dog from HsN8. That is also why we humans have to get a flu vaccine every year- the strains change. While we have memory cells from the strain we were vaccinated against last year, they don’t recognize the flu strain for this year.

The reason that we often give a series of vaccines to adult dogs and cats is that we are trying to get enough memory cells to provide long-lasting protection. In puppies and kittens, there is another reason as well.

Graph showing that memory cell response is more protective than antibody response in the immune system
Memory Cell vs Antibody Immunity

From this graph, you can see that memory cells protect the individual much better than antibodies over the long run. There is no easy way to measure the magnitude of memory cell response to a particular antigen. We can measure antibodies for certain diseases. If an individual has been vaccinated or infected by a particular pathogen, they may have undetectable antibody levels in their blood. But they may still be immune or protected from serious disease due to memory cells.

How Baby Animals Are Protected From Disease

A vaccinated bitch or queen can provide some protection to the puppies or kitten through the milk or in the womb. These are maternal antibodies. The mom can also acquire antibodies by recovering from infection. Maternal antibodies are ready to fight disease right away, just like antibodies produced by the individual.  This is called passive immunity because the puppy or kitten didn’t produce the antibodies themselves.

Unfortunately, maternal antibodies only last 8-12 weeks. At some point, the puppy or kitten is going to be susceptible to some pretty horrific diseases such as parvo or panleukopenia. We can’t have that! Even though these diseases are potentially survivable, they are NOT CHEAP to treat. Plus, these little babies are SICK. And there is absolutely no need for any animal to have to suffer this way because we have vaccines!   

How Vaccines Work

Vaccines are either little parts of a pathogen or an inactivated (killed or attenuated) pathogen that trigger the immune system without actually causing disease. Vaccines fool the immune system into thinking there is a real disease, so it reacts the same way. There are essentially 3 types of vaccines: modified live vaccines (MLV), killed vaccines, and recombinant vaccines.  

Modified Live Vaccines

Modified live vaccines are exactly what it sounds like- vaccines made from the actual pathogen but changed in such a way that they don’t cause disease. Because the MLV is so close to the actual disease, they are excellent vaccines, stimulating a massive immune response. They typically last a lot longer than other types of vaccines as well. Some examples of modified live vaccines in veterinary medicine are the canine distemper/parvo vaccines, and feline rhinotracheitis/panleukopenia/calicivirus vaccines.

Killed Vaccines

Killed vaccines are also exactly what they sound like- the pathogen is treated typically to heat, enough to kill the pathogen while keeping the antigen proteins intact. Injecting a bunch of dead pathogen will not stimulate the immune system though. So substances called adjuvants are added to killed vaccines in order to make the immune system recognize the pathogen and react. These vaccines typically don’t last as long as MLV, because they don’t replicate in the body.

Furthermore, in cats, the adjuvants can cause a very aggressive type of cancer called vaccine-associated sarcoma. Thankfully, there are other options available for cats so they never have to receive an adjuvanted vaccine. Unfortunately, these vaccines are more expensive than the killed ones and some practices are still risking their patients’ lives by administering these vaccines. All of our feline vaccines are non-adjuvanted, always have been, and always will be.  Our canine rabies vaccine is a killed vaccine.

Recombinant vaccines are pretty slick. They take the part of the pathogen that the immune system recognizes and splice it into a harmless live virus, usually canarypox, that can replicate inside the body. The immune system reacts to these vaccines just like an MLV, with usually long-lasting immunity.   Our feline leukemia vaccine is a recombinant vaccine, also without an adjuvant.

How Puppies and Kittens are Protected from Disease

If vaccines are so great at preventing disease, why do puppies and kittens need so darn many of them? The answer surprisingly is not that their immune system doesn’t work that well due to immaturity. A baby animal’s immune system works just as well as an adult animal’s. The problem is those darn maternal antibodies. Remember those? Well, turns out that maternal antibodies are so effective at destroying antigens, they destroy our vaccines too! I’ll be honest with you, most of the DAPP and FVRCP vaccines we administer are going to be destroyed and are useless to the puppy or kitten.  

Graph showing the window of susceptibility of parvo in a puppy during their vaccine series
Window of Susceptibility of Puppies to Parvovirus

The problem is that we never know exactly when those maternal antibodies are going to wear off in each individual animal. It could be 8 weeks, it could be 12 weeks, and it can vary even between litter mates. And if the animal doesn’t have maternal antibodies and it isn’t vaccinated, it’s going to be susceptible to disease.

Alrighty then, we’ll just vaccinate the puppy or kitten every 3 weeks until such an age that we KNOW there will be no more maternal antibodies to destroy our vaccine. There is a very small window of susceptibility as shown in this graph, where the maternal antibodies can destroy the vaccine, but they aren’t enough to protect the pet. Thankfully, that is a small window, and with vaccines every 3 weeks, it is unlikely that the pet will get sick. However, it does show just how important it is to keep with the vaccine schedule. 

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